Thermal projection device

ABSTRACT

The invention relates to a device and a method for control of the operation of a thermal projection torch ( 12 ), characterized in that the characteristics of the jet ( 16 ) and the temperature of the deposit ( 24 ) on the piece ( 22 ) are measured by means of a camera ( 54 ) and a combined pyrometer ( 70 ). The correction to be made to the supply parameters of the torch ( 12 ) are deduced therefrom and the corrected parameters are transmitted to the unit ( 30 ) controlling the torch ( 12 ).

TECHNICAL FIELD OF THE INVENTION

The invention relates to the coating of surfaces by thermally sprayingmolten materials with the aid of a thermal spray torch, referred tobelow as a torch, and more particularly to a thermal spray instrumentwhich has a device for monitoring and managing the thermal spraying.

PRIOR ART AND OBJECT

Thermal spraying is a well-known method for coating a solid surface witha material which has a high melting point. It consists in melting thematerial in a high-speed hot gas flow directed onto the surface, withthe gas flow atomizing the material as small molten droplets andentraining the droplets to the surface, the droplets still in the moltenstate being flattened onto the surface, the droplets adhering to thesurface and solidifying in contact with it. The gas flow loaded withmolten droplets is referred to as a jet. The coating is obtained bysuccessive passes by moving the jet with respect to the surface.

Thermal spraying can be used for various purposes: decoration, thermalbarriers, protection against oxidation or chemical corrosion, reapplyingmaterials, enhancing mechanical characteristics of the surface, inparticular abrasion resistance, etc.

The spray material may be a pure metal such as molybdenum or titanium, ametal alloy such as NiCr, NiAl, NiCrAlY, a ceramic such as Cr₂O₃ orZrO₂, a carbide such as WC or Cr₃C₂, or a cermet such as Cr₃C₂/NiCr.

Various methods of thermal spraying are known, each of which uses aparticular torch.

So-called “flame” thermal spraying consists in producing a flame by thecombustion of gases with a high calorific value, such as acetylene andoxygen, the rise in temperature producing a high-speed gas stream intowhich the material to be sprayed is injected in the form of powder orwire. The material melts in contact with the flame, is atomized as smallmolten droplets in the flow of hot combustion gases and is entrained bythis flow in order to form the jet.

So-called “arc wire” thermal spraying consists in producing an electricarc between two wires of the material to be sprayed, and in passing aflow of neutral gas, such as argon Ar, onto the electric arc at a highspeed. The material of the wires liquefies in the presence of theelectric arc, is atomized as small molten droplets in the flow of hotcombustion gases and is entrained by this flow in order to form the jet.

So-called “arc plasma” thermal spraying consists in producing heat bysustaining an electric arc in a flow of plasma-generating gas, with theplasma formation leading to a significant rise in the temperature of thegas, and in the powder material to be sprayed being injected into thisflow, this powder being fluidized and transported by an inert so-called“carrier” gas. The combination consisting of the plasma-generating gas,the carrier gas and the material melted as fine droplets in contact withthe plasma-generating gas forms the jet.

The jet has the form of a divergent cone at the outlet of the torch.Because of the high temperatures involved, a torch degradesprogressively as it is used, this degradation leading to drifts in itsoperation as well as deformations and deviations of the jet. In certaintypes of arc plasma torch, the powder injection takes place transverselyto the plasma-generating gas flow at the outlet of the torch, whichleads to a normal deviation of the jet.

The torch is conventionally small so that it can be moved convenientlyin front of the surface to be covered. This torch is connected to acontrol unit which supplies it with electric current and the variousingredients needed for its operation. The term ingredients is intendedto mean the gases and the materials as described above.

The quality criteria of a deposit formed by thermal spraying areconventionally its hardness, its adhesion at the coated surface, itsporosity, the absence of cracks, the unmolten fraction and, in the caseof metallic materials, its oxide level. The term “unmolten fraction” isintended to mean the proportion of the material constituting the depositwhich has not passed through the molten state. Attention is also paid tothe efficiency of the spraying, that is to say the proportion of thematerial used which will actually constitute the deposit, the rest ofmaterial being lost on the walls around the thermal spray installation.

The quality of the deposit and the efficiency of the depositingoperation clearly depend not only on the material employed but also onthe settings and type of the torch. The material flow rate, for examplein grams per minute, is clearly a parameter common to all the torches.In the case of flame spraying, it is also necessary to set the flowrates of combustible and oxidant gases expressed, for example, in litersper minute. In the case of arc wire spraying, it is also necessary toset the arc intensity in amperes and the gas flow rate. In the case ofarc plasma spraying, it is also necessary to set the arc intensity, theflow rate of plasma-generating gas and the flow rate of carrier gas.

It is difficult to obtain a constant deposition quality because thetorch and its supply of ingredients are subject to inaccuracies anddrifts over time, which clearly affect this quality. Before coatingoperations are carried out, it is necessary to test the torch on samplesand adjust the settings if so required. But this is not enough. Duringthe coating operations, it is also necessary to carry out checksperiodically on the basis of samples and modify the settings, or changethe torch if so required. This is because a torch degrades progressivelyas it is used, especially in its hot parts such as the injection nozzle,and these degradations can make the characteristics of the torch driftand make the jet deform or become displaced. These checks should befrequent so that the appearance of a drift can be detected early enoughand the settings of the torch can be modified before the quality of thedeposit has itself drifted outside the acceptable limits. These checksand these adjustments clearly take time and reduce the productivity ofthe installation. In the case of prolonged coating operations, it mayfurthermore be necessary to interrupt it in order to check the torch orthe quality of the deposit and, if need be, change the setting of thetorch or replace it.

A first object to be achieved is to verify that the torch is capable ofproviding a deposit whose characteristics comply with what was intended,this verification necessarily having to be carried out in realtimeduring a thermal spraying operation, and also to correct the operationof this torch in realtime when drifts are found.

A second object is to achieve these results by inexpensive means.

A third object is to stop the torch automatically when it is no longercapable of operating normally and consequently runs the risk ofproducing defective coatings.

DESCRIPTION OF THE INVENTION

In order to achieve this object, the invention provides a thermal sprayinstrument having a thermal spray torch, the torch being capable ofspraying a jet along its geometrical axis, the jet consisting of a gasflow at elevated temperature loaded with molten particles of thematerial to be sprayed, the instrument having a control unit supplyingthe torch with ingredients by applying the supply parameters which arecommunicated to it, the instrument having a computer communicating thesupply parameters to the control unit by means of a unit-computerconnection, the instrument having sensors capable of monitoring themovements of the torch, the sensors being capable of transmittinginformation about the operation of the torch to the computer, thistransmission being carried out by means of the sensors-computerconnection.

Such an instrument is noteworthy in that:

-   -   a. the computer has software for realtime analysis of the        information, in order to repetitively deduce therefrom the        measurement of at least one so-called “spray” parameter so as to        find when this measurement is stabilized, in order to “process”        this spray characteristic, that is to say in order to calculate        a new supply parameter value and transmit it to the control unit        when the measured value of the spray characteristic is outside a        preestablished so-called “acceptable” range of values specific        to the spray characteristic being processed, this new value of        the supply parameter being suitable for returning the spray        characteristic to its acceptable range.    -   b. the sensors have an optical pyrometer capable of remotely        measuring the thermal radiation at the surface of some article        to be coated, which is positioned in front of the torch, the        pyrometer having a narrow field, the pyrometer being positioned        so that the field comes as close as possible to the jet (16) on        the article, but without interfering with this jet, the        pyrometer also being capable of periodically transmitting the        temperature measurement to the computer by means of the        sensors-computer connection.    -   c. the computer is capable of correcting the temperature        measurement as a function of the emissivity coefficient of the        coating, this corrected temperature called deposite temperature        T constituting the spray characteristic.

Such an instrument makes it possible to effectively regulate theresidual stresses of the deposit, because these residual stresses havebeen found to depend strongly on the temperature T of the deposit.

Advantageously:

-   -   d. the sensors have a camera capable of periodically providing        the computer with the information in the form of digital images        of the jet as seen in profile over a part of its length,    -   e. the computer also measures from the image and processes the        width L of the jet, L also constituting a spray characteristic,        an order of priority being defined in the processing of the        spray characteristics, the processing of the temperature being        given priority however, the camera being capable of observing        the jet with a resolution at least equal to 0.5 mmm, L being        proportional to the standard deviation of the distribution of        the luminance of the jet along a geometrical line transverse to        the jet.

Such an instrument makes it possible also to regulate the hardness ofthe coating, because this hardness has been found to depend strongly onthe width L of the jet.

Also advantageously:

-   -   d. the sensors have a camera capable of periodically providing        the computer with the information in the form of digital images        of the jet as seen in profile over a part of its length,    -   e. the computer also measures from the image and processes the        position P of the jet, P also constituting a spray        characteristic, an order of priority being defined in the        processing of the spray characteristics, the processing of the        temperature T still being given the highest priority, the camera        being capable of observing the jet with a resolution at least        equal to 0.5 mmm, P being, within a constant value P₀, the mean        value of the distribution of the luminent of the jet along a        geometrical line transverse to the jet.

Such an instrument makes is possible also to regulate the level ofcracks of the coating, because it has been observed that this level ofcracks depends strongly on the position P of the jet.

Also advantageously:

-   -   d. the sensors have a camera capable of periodically providing        the computer with the information in the form of digital images        of the jet as seen in profile over a part of its length,    -   e. the computer measures from the image and processes the        maximum luminance Imax of the jet, Imax also constituting a        spray characteristic, an order of priority being defined in the        processing of the spray characteristics, the processing of the        temperature T still being given the highest priority.

Such an instrument makes is possible also to regulate the level ofoxides of the coating, because this level of oxides has been found todepend strongly on the maximum luminance Imax of the jet.

The camera will advantageously be of the CCD type, the effect of thecharge accumulation in the pixels of the matrix being to filter thehigh-frequency vibrations of the jet, the result of which is to improvethe estimate of the characteristics of the jet, and consequently toregulate the thermal spraying better. The measurements may be carriedout simply in the visible light spectrum. In the case of applicationswhich require very good regulation of manufacturing processes, forexample in the aeronautical and space industries, a camera giving imagesof the jet with a resolution at least equal to 0.1 mm will be selectedin order to improve the regulation of the spray characteristics, andconsequently the characteristics of the deposits which are formed.

The camera, the pyrometer and the computing means employed are readilyavailable on the market and inexpensive, so that the second object isachieved.

The invention will be understood more clearly and the advantages whichit offers will become more readily apparent on studying the followingdetailed description of some numerical exemplary embodiments and theappended figures.

DESCRIPTION OF THE FIGURES

FIG. 1 schematically represents a thermal spray installation.

FIG. 2 illustrates an arc plasma thermal spray torch with transverseinjection of the powder material to be deposited, the jet being seenalong the geometrical axis referenced 56 in FIG. 1.

FIG. 3 illustrates the onboard sensors with the CCD camera and theoptical pyrometer.

FIG. 4 illustrates the optical pyrometer and its sight.

FIG. 5 gives an example of a relational diagram of the information ofthe database.

FIG. 6 illustrates the images which are processed by the computer.

FIG. 7 gives a synthetic example of an algorithm for performing thefunctions of the computer.

DETAILED DESCRIPTION

An installation of the thermal spray type, and the monitoring devicewhich is associated with it according to the invention, will bedescribed first of all.

Reference will firstly be made to FIG. 1. The thermal spray instrument10 has a thermal spray torch 12 of geometrical axis 14 which sprays ajet 16 along this geometrical axis 14, consisting of a hot gas flowloaded with droplets of the molten material to be sprayed: metal, metalalloy, ceramic or cermet. The jet 16 is divergent and is conventionallyin the form of an axisymmetric cone centered on the geometrical axis 14.A very bright flame 17 sometimes emerges from the torch 12 in thevicinity of the apex of the cone formed by the jet 16. In the case ofplasma torches, this flame 17 may reach a temperature of 8000° K. Thejet is still bright beyond this flame 17, but this luminosity is now dueessentially only to the droplets of molten material. The jet 16 isnormally centered on the geometrical axis 14. Because of the hightemperatures employed in the torch 16, and in spite of the coolingdevices integrated in these torches 16, the torches 16 degradeparticularly by erosion during their use, and these degradations canmodify the characteristics of the jet 16, deform the jet 16 or make thegeometrical axis 14 deviate.

Reference will now be made to both FIGS. 1 and 2. The torch 12 is an arcplasma torch of the type using transverse injection and has an injectorwith a geometrical axis 20 perpendicular to the geometrical axis 14 ofthe torch, the powder material to be sprayed being injected into the jet16 by this injector 18 with the aid of a so-called carrier gas, thisinjection taking place just at the outlet of the torch 12 in thevicinity of the apex of the cone formed by the jet 16, this injectiontaking place transversely to the jet 16 and leading to a deviation ofthe jet 16 in the opposite direction to the injector 18, the jet 16 thenmoving away normally from the geometrical axis 14.

The droplets of molten material sprayed by the jet 16 arrive at a highspeed and are flattened onto the surface of the article 22 to becovered, in order to form the intended deposit 24 there bysolidification and adhesion. This deposit 24 normally consists ofsuccessive layers, with the torch 12 sweeping repeatedly over thesurface of the article 22. The surface of the article 22 exposed to thejet 16 at a given time will be referenced 26.

The thermal spray instrument 10 also has a conduit 28 and a control unit30, this control unit 30 supplying the torch 12 with ingredients bymeans of the conduit 28, the supply consisting in providing the torch 12with the ingredients required for it to operate. The flow rates of theseingredients will be referred to as “supply parameters”.

In the case of an arc plasma torch, the essential supply parameters ofthe torch are:

-   -   the electric arc current I and the voltage V which results        therefrom;    -   the flow rate of each plasma-generating gas, such as hydrogen H₂        and argon Ar, expressed for example in liters per minute, the        liters being considered at atmospheric pressure;    -   the material flow rate Dm, expressed for example in grams per        minute;    -   the carrier gas flow rate, also expressed for example in liters        per minute, this gas usually being argon and being denoted        Ar_(carrier),

The torch 12 is cooled by circulation of water.

The torch may be hand-held, for example in order to repair civilengineering structures made of metal. It is most often used on apreferably robotized installation 40 which holds, positions and movesthe torch 12 relative to the article 22 to be processed. Theinstallation 40 will preferably have a robot arm 42 supporting the torch12, as well as a fixed or pivoting article holder 44 which holds thearticle 22 in front of the torch 12.

Reference will now be made to FIG. 3. According to the invention, thespray instrument 10 has onboard sensors 52 attached to the torch 12 soas to monitor it in its movements during the thermal spraying, theseonboard sensors 52 thus remaining in a constant relative position withrespect to the torch 12.

The onboard sensors 52 firstly consist of a CCD camera 54 which canprovide digital images of the jet 16, these being taken transversely orperpendicularly to this jet 16. When the jet 16 has a flame 17 at itsstart, the camera 54 is positioned in order to provide an image of thejet 16 beyond the flame 17, that is to say downstream of this flame 17,so that the image of the jet 16 is not obscured by the light of theflame 17. The geometrical imaging axis of the camera 54 will bereferenced 56. Preferably, but not necessarily, the camera 54 isarranged beside the torch 12 and sights the jet 16 by means of animaging mirror 58 arranged at 45° in front of the camera 54, thisimaging mirror 58 deviating the geometrical axis 56 of the CCD camera by90° and allowing the CCD camera 54 to see the jet 16.

Such an arrangement thus makes it possible to free the space between thetorch 12 and the article 22 as much as possible.

The camera should have a sufficient resolution to be able to pick up 0.5mm details on the jet 16. This is because such a resolution is necessaryin order to detect and measure a 0.5 mm deviation of the jet. In thecase of aeronautical applications, this resolution should actually be atleast equal to 0.1 mm in order to regulate the characteristics of thedeposit with a sufficient precision. In this example, the camera has aCCD (Charge Coupled Device) matrix of 640×480 pixels with an exposuretime ranging from {fraction (1/30)} second to {fraction (1/2000)} secondin order to observe jets of very different light intensity with asufficient precision and without saturating the pixels of the matrix.The sensitivity of the CCD camera may be limited to the visiblespectrum. A black-and-white camera is sufficient, although it is alsopossible to use a color camera. Such a camera is readily available onthe market at a low price. It is sufficient for it to have adequatestability against the heat released during the thermal spraying.

In the event that the torch 12 has an injector 18 for powder material tobe sprayed, the camera 54 is positioned in order to see the jet 16 alonga geometrical axis 56 substantially orthogonal to the geometrical axis20 of the injector 18, this position making it possible to optimallyvisualize the deviation of the jet 16 due to this injection mode, thisposition consequently making it possible to monitor the deviation of thejet 16 more effectively.

Compared with conventional acquisition devices such as a linear array ofphotodiodes, the CCD camera has the following advantages.

-   -   Smoothing of the high-frequency vibrations of the jet, which is        obtained by the effect of charge accumulation in the pixels of        the CCD matrix, this accumulation taking place in proportion to        the light which they receive, with vitiation of the measurements        and the introduction of instabilities into the management of the        torch being avoided by this smoothing. Specifically, the sensors        integrate the light received during the exposure time, so that        the variations in the luminance of the jet 16 due to these        vibrations are divided by the ratio d/t, d being the exposure        time and t being the period of the vibrations of the jet. In a        conventional device, it would have been necessary to provide        lowpass electronic filters on each of the photosensitive        elements, which would increase their bulk and limit the number        of them.    -   High resolution in a small volume, namely a few cubic        centimeters.    -   Inexpensive device making it possible to take the images and        transmit them to the computer by standard commercially available        means.

The onboard sensors 52 also consist of an optical pyrometer 70 with ageometrical axis 72, which remotely measures the thermal radiationemitted by a so-called “measurement” surface 73, the measurement surface73 having small dimensions along the geometrical axis 72. The pyrometer70 is directional and it can be aimed at the article 22 as close aspossible to the spraying zone 26 but without interfering with thisspraying zone 26, that is to say the measurement zone 73 is close oradjacent to the spraying zone 26 but does not interfere with thisspraying zone 26. In other words, the pyrometer 70 has a narrow fieldand it is positioned so that the field comes as close as possible to thejet 16 on the article 22, but without interfering with this jet 16. Withthis arrangement, the very bright jet 16 remains outside the field ofthe pyrometer, and in particular the measurement zone 73, so that thepyrometer 70 receives the thermal radiation of the deposit 24 but notthe light radiation of the jet 16, which could vitiate the measurementof the temperature of the deposit. In order to facilitate positioning ofthe pyrometer 70, it preferably has a laser sight 74 projecting a lightspot onto the measurement zone 73.

The measurement of the thermal radiation is conventionally taken in theinfrared range, that is to say in the electromagnetic radiation bandextending from 0.8 μm to 14 μm. In the particular case of arc plasmatorches, this will preferably be done in the 8 μm-14 μm band in order toobtain a stable, precise and inexpensive measurement. This is because ithas been found that with this type of torch, ionization of the watervapor H₂O and the carbon dioxide gas CO₂ contained in the air takesplace in the vicinity of the jet 16, this ionization leading toabsorption of the infrared radiation in the 0.8 μm-3.46 μm and 4.78 μm-8μm bands for water vapor and in the 4.2 μm-4.5 μm band for carbondioxide gas. It has been found that temperature measurements takenwithout excluding these absorption bands are unstable and affected bybackground noise which makes them difficult to use. It is thereforepreferable to take the measurement in the 8 μm-14 μm band, this bandbeing wide enough so that the pyrometer 70 can be equipped with aninexpensive filter. It is also possible to take this measurement in the3.46 μm-4.2 μm or 4.5 μm-4.78 μm bands, but these are narrow and it isthen necessary to equip the pyrometer 70 with high-performance andtherefore expensive narrowband filters.

An example of a sight 74 is illustrated in FIG. 4. The sight 74 projectsa narrow laser beam 78 along the geometrical axis 72 of the pyrometer70. To this end, the sight has a diode laser 76 arranged beside thepyrometer 70, the diode laser 76 emitting a laser beam 78 forward fromthe pyrometer 70, parallel to its geometrical axis 72, the laser beam 78being brought into the geometrical axis of the pyrometer 72 by aconventional set of two mirrors 80 and 82, the second mirror 82 beingsemi-reflective and positioned along the geometrical axis of thepyrometer 72. With such an arrangement, the setting of the sight doesnot depend on the distance between the pyrometer 70 and the surface ofthe article whose temperature is to be measured.

It will be noted that the pyrometer 70 gives an exact measurement of thetemperature only for perfect black bodies. In reality, it is necessaryto take into account the emissivity coefficient E of the material whosetemperature is being measured, this emissivity coefficient E lyingbetween 0 and 1, the real temperature T being related to the temperatureT_(obs) observed by the pyrometer by the following relation:T≈C/λ[ln(E)+C/λ.T _(obs)]1-273 with C=0.00144T_(obs) being the absolute temperature expressed in degrees Kelvin and Tbeing expressed in degrees Celsius for convenience.

The measured temperature can thus be calculated by analog or digitalmeans.

Reference will again be made to FIG. 3. The onboard sensors 52 arearranged inside a closed compartment 90 which protects them againstexternal agents, although this compartment 90 does have openings 92allowing the camera 54 to see the jet 16 and allowing the pyrometer 70to see the surface of the article 22, this compartment 90 having acompressed air supply 94, this compressed air emerging through openings92 and forming an obstacle to the ingress of dust and droplets into thecompartment during operation of the torch 12, such dust and dropletsbeing liable to deposit on the sensors 52 and in particular foul theiroptical components.

Reference will again be made to FIG. 1. The thermal spray instrument 10also has a computer 100 connected by the connection 110 to the onboardsensors 52, that is to say to the camera 54 and to the pyrometer 70. Bymeans of this connection 110, the computer 100 is capable of receivingin realtime the digital images 112 coming from the camera 54, as well asthe temperature readings 114 coming from the pyrometer 70. The computer100 is also connected to the control unit 30 by means of the connection120. By means of this connection 120, the computer 100 is capable oftransmitting the supply parameters to the control unit 30 in realtime.Also by means of this connection 120, the computer is capable ofreceiving the supply parameters from the control unit 30 in realtime,for example the voltage V of the arc in the case of a plasma torch. Theterm “realtime” is intended to mean the value of the information to beapplied as soon as it is received, or the current value of theinformation which will be transmitted. The computer 100 may be acommercially available microcomputer equipped with suitable connectionmeans so that it can be connected to the connections 110 and 120respectively leading to the sensors 52 and to the control unit 30, thiscomputer 100 also needing to have enough power to perform the processingoperations at the appropriate frequency.

Reference will now be made to FIG. 5. The computer 100 also has adatabase 130 containing the information needed in order to monitor andmanage the thermal spraying. In this example, the information is groupedas models, each model providing the information required in order tomanage a depositing operation by thermal spraying with a torch, adeposit composition and specified deposit characteristics.

The model firstly contains the information designating it, that is tosay:

-   -   the torch model being used;    -   the deposit composition to be formed;    -   the deposit characteristics to be obtained;        this information making it possible to designate it        unequivocally and select the appropriate model from the        database, for example by a simple multicriterion search.

The model contains general information:

-   -   emissivity coefficient for calculating the exact temperature        from the measurement given by the pyrometer;    -   image acquisition period of the CCD camera;    -   Number of images per batch;    -   Background noise level;    -   Stability threshold level of the jet.

A model contains the spray characteristics which are to be taken intoaccount and which need to be managed, namely:

-   -   I_(max): maximum luminance of the jet;    -   L: width of the jet;    -   P: position of the jet;    -   T: temperature of the deposit.

For each of these spray characteristics, the model also contains:

-   -   an order of priority;    -   a so-called “acceptable” range defined by a minimum value and a        maximum value;    -   and a so-called “optimum” range also defined by a minimum value        and a maximum value, the optimum range obviously being included        within the acceptable range of the corresponding spray        characteristic.

The model contains the supply parameters which need to be altered inorder to manage the spray characteristics. These parameters clearly varywith the torture model being used. For example, in the case of a plasmatorch:

-   -   I: Arc intensity;    -   Ar: flow rate of plasma-generating argon;    -   H₂: flow rate of plasma-generating hydrogen;    -   Ar_(carrier): flow rate of carrier argon.

For each supply parameter, the model contains:

-   -   an initial value to be transmitted to the control unit when a        thermal spraying operation is being started;    -   an order of priority by being applied uniformly by default to        the supply parameter for all the spray characteristics on which        it has an effect.    -   a normal operating range of the torch, expressed by a minimum        value and a maximum value, this range optionally also expressing        the validity limits of the equation,    -   a correction step size,

It should be noted that in certain more complex cases, the order ofpriority and the normal operating range should be specified for eachequation involving the supply parameter in question.

Lastly, the model gives the statistical relations between the spraycharacteristics and the supply parameters of the torch in the form of asystem of equations, each equation of which is a polynomial of the form:

spray characteristics

-   -   =F(supply parameter)    -   =K+Σ_(i)c_(i)p_(i)+Σ_(jk)c_(jk)p_(j)p_(k)        in which:    -   K is a positive or negative constant;    -   c_(i) is a positive or negative coefficient associated with the        supply parameter i;    -   p_(i) is the current value of the supply parameter i;    -   c_(jk) is a positive or negative coefficient associated with the        product of two supply parameters j and k.

In practice, each polynomial is linear and sometimes of degree 2. Higherdegrees are conceivable, but it then becomes difficult to estimate therelations and the degree of dependence between the spray characteristicsand the supply parameters.

These relations are clearly statistical, and moreover established bylaboratory studies. They are valid, with an acceptable dispersion, onlywithin a range of values that are specified for each supply parameter.For instance, the so-called “normal operating” range may correspond:

-   -   either to limitations of the torch;    -   or to limitations of the validity of the equation corresponding        to a degree of dependence deemed acceptable between the spray        characteristic in question and the supply parameters on which it        depends.

Preferably.

-   -   The order of the equations gives the order of priority in which        the spray characteristics should be corrected.    -   The order of the supply parameters in each equation gives the        order of priority in which it is necessary to modify the supply        parameters in order to correct the corresponding spray        characteristic.

We will now define the spray characteristics more precisely, andreference will be made to both FIGS. 1 and 5.

The maximum intensity I_(max) of the jet is the maximum luminance of thejet 16, this maximum luminance of the jet conventionally being at thecenter of the jet 16 as seen laterally from the outside and downstreamof any flame 17 which may emerge from the torch 12. The luminance is aphysical quantity which can be expressed in watts per square meter persteradian (W/m²/sr). The maximum light level of the pixels of the points112 as given by the matrix of the CCD camera 54 will preferably beadopted. This light level is common to the known image standards such asa bitmap, GIF, PSD, etc. It is conventionally encoded over eight bitsand consequently scales from zero to 255. If the CCD camera 54 beingused provides color images, that is to say in red—green—blue additivetrichromicity, then the maximum light level of the color green maysimply, but not necessarily, be adopted, this color green being the onemost resembling the behavior of a black-and-white CCD camera.

The width L of the jet is a quantity selected in order to characterizethe width of the jet 16. Since the edges of the jet 16 are disperse andnot clearly defined, a quantity proportional to the standard deviation aof the distribution of the luminance of the jet in the width directionof the jet will preferably be adopted. In practice, the standarddeviation a of the distribution of the light levels of the pixels overthe image 112 of the jet in the width direction of the jet 16 a on thisimage 112 will be adopted, for example along a row of pixels 154perpendicular to the position 14 a of the geometrical axis 14 of thetorch on the image 112. For example, L=2σ expressed in millimeters willbe adopted.

The position P of the jet is the position of the jet with respect to thegeometrical axis 14 of the torch 12. Since the edges of the jet 16 aredisperse and not clearly defined, P will preferably be the average ofthe distribution of the luminance of the jet, also in the widthdirection of the jet 16 a on the image 112, for example and as beforealong a row of pixels 154 perpendicular to the position 14 a of thegeometrical axis 14 of the torch on the image 112.

It has been found that the distribution of the light levels of thepixels in the width direction of the jet 16 a on its image 112approximately follows the well-known Gaussian law in the form:I=G(I _(max) , P, σ)=I _(max).exp(−(x−(P+P ₀))²/σ²)/2π, with:

-   I=light level of the pixels in the width direction of the jet;-   x=position of the pixel;-   P₀=position 14 a of the geometrical axis 14 of the torch on the    image 112, this position 14 a being found easily by fitting a rod in    the nozzle of the torch 12 and by taking an image 112 of this rod.

It is consequently preferable to process this additional information anddeduce Imax, P and a from the estimate of the Gaussian law G of thedistribution of the light levels of the pixels, as before in the widthdirection of the jet 16 a on the image 112, and as before along a row ofpixels 154 perpendicular to the position 14 a of the geometrical axis 14of the torch on the image 112, in which case this estimate may beobtained by the well-known so-called “least squares” method.

In order to reduce the effect of stray light and light reflections ofall kinds around the thermal spray installation, this stray light beingliable to cause a diffuse shadow 156 on the images 112, on either sideof the image of the jet 16 a, as well as bright spots 158 due toreflections, this shadow and these bright spots 156 being liable tonon-repetitively vitiate the estimates of the characteristics of thejet, it is preferable to take into account only the pixels whose lightlevel is higher than a threshold value referred to as the “backgroundnoise level”. This threshold value is easy to determine by separatelyanalyzing a few test images. In practice, it is equal to 4 or 5 on ascale ranging from zero to 255 for the light levels on the images 112.

The temperature T of the deposit is the temperature as measured by thepyrometer 70 and corrected as a function of the emissivity of thedeposit.

The CCD camera should have a sufficient resolution in order to measurethe width L of the jet and its position P with a reliability of 0.5 mmin ordinary applications and 0.1 mm in aeronautical applications. Thismeans that the measurement should be repetitive and that they can detectdifferences of respectively 0.5 mm and 0.1 mm in the variations of thequantities being measured. The camera used here has a matrix of 640×480pixels.

Reference will now be made to both FIGS. 1 and 6. The computer 100 isequipped with monitoring software which accesses the database 130 inorder to perform the following functions:

-   -   Giving the control unit the initial values of the supply        parameters when a depositing operation is being started.    -   Acquiring the images 112 coming from the CCD camera N times per        second, and grouping them into batches of N1 images, and        acquiring a temperature measurement 114 from the pyrometer at        the end of each image batch.    -   For each image, calculating the jet characteristics being used,        on the basis of the image pixels which are selected from a pixel        row 154 transverse to the image of the jet 16 a and whose light        level nl is higher than that of the background image.        -   If x denotes the rank of the pixel along the pixel row 154,            P₀ the position 14 a of the geometrical axis 14 of the torch            on the image 112, nl the light level of the pixels and n the            number of pixels, then Imax, L and P can be calculated by            the following formulae:            -   I_(max)=maximum (nl)        -   p=average of x=Σx.nl/Σnl−P₀        -   L=2×standard deviation=2σ=2.square root[Σ(x.nl)²/n−(P+P₀)²]        -   Imax, P and L preferably deduced from a Gaussian law            established, for example, by the well-known so-called “least            squares” method on the basis of the distribution of the            light levels nl of the pixels along the row of pixels, this            Gaussian law being of the form            I_(max).exp(−(x−(P+P₀))²/σ²)/2π

This is a preferred embodiment of the invention, in which the position Pof the jet is estimated with respect to a reference position P₀corresponding to the geometrical axis of the torch 12. It will beunderstood that any other estimate of P, calculated to within a constantvalue, will give the same result. It will be sufficient to change theconstant term in the equation giving the spray characteristic P as afunction of the supply parameters which have an effect on P.

For ease of expression, the use of the spray characteristics with a viewto calculating new supply parameters and transmitting them to thecontrol unit will be referred to as “processing”. In this context, thecomputer 100 fulfills the following functions.

-   -   For each batch: verifying that the jet 16 is stabilized by        verifying that the differences in the jet characteristics        between the images of the batch are at most equal to the        stability threshold level of the jet.    -   For each image batch relating to a jet 16 assessed as being        stabilized:        -   calculating the spray characteristics by averaging the            measurements of Imax, L, P and by correcting the temperature            T as a function of the emissivity of the surface being            measured.        -   Finding the most important spray characteristic which has            drifted outside its predefined acceptable range, and            determining and transmitting to the control unit 30 the            supply parameter to be corrected as well as its new value,            which are suitable for returning the spray characteristic to            its acceptable range.        -   Emitting a warning signal and transmitting a stop            instruction to the control unit when it is not possible to            return a spray parameter to its acceptable range without            making all the supply parameter depart from their predefined            normal operating ranges.        -   When all the spray characteristics are each in their            acceptable range: finding the most important spray            characteristic lying outside its predefined optimum range,            and determining and transmitting to the control unit the            supply parameter to be corrected as well as its new value,            which are suitable for returning the spray characteristic to            its optimum range. For the sake of simplicity, and although            it is not obligatory, the spray characteristics will be            processed with the same orders of priority.

FIG. 6 gives an example of an algorithm for fulfilling these functionsin a synthetic form. It is synthetic since it only gives the generallogic of the monitoring and management of the operation of the torch,because the estimation of the spray characteristics, the choice of thespray characteristics and the corresponding supply parameters to becorrected, as well as the calculation of this correction, can beobtained by straightforward programming.

We will now study some numerical exemplary embodiments of the presentinvention. The torch employed is a thermal plasma spray torch withexternal injection and, more specifically, the model F4 MB sold by theSwiss company whose corporate name is Sulzer Metco. In these examples,the torch is used in substantially common operating ranges, so that thesame equations can be used.

The general information is as follows: Image acquisition frequency = N =100/second Number of the images per batch = N1 =  10 Background noiselevel =  5 Jet stability level =  1%

It should be noted that the images and the temperature measurements areavailable directly at the ports of the computer in the installationwhich was produced by the Inventors.

The values of the spray characteristics I_(max), P, L and T are given bythe following equations:I  max  = −45.2957 − 1.51175 * Ar + 38.2083 * H₂ + 0.234739 * I − 8.94 * Ar_(carrier) − 0.39724 * ArH₂ − 0.00272557 * Ar * I + 1.04463 * Ar * Ar_(carrier) + 0.0170028 * H₂ * I − 6.46563 * H₂ * Ar_(carrier) − 0.0231932 * I * Ar_(carrier)P = −7.85889 + 0.0795898 * Ar − 0.0244141 * H₂ + 0.00776811 * I + 2.22168 * Ar_(carrier) − 0.000712077 * ArH₂ − 0.0000521573 * Ar * I − 0.0266113 * Ar * Ar_(carrier) − 0.000616599 * H₂ * I + 0.10376 * H2 * Ar_(carrier) − 0.000998757 * I * Ar_(carrier)L = 17.9632 − 0.30375 * Ar − 0.377083 * H₂ − 0.00725 * I − 0.025 * Ar_(carrier) + 0.0107292 * Ar * H₂ + 0.000126136 * Ar * I + 0.04675 * Ar * Ar_(carrier) − 0.0000473485 * H₂ * I + 0.0395833 * H₂ * Ar_(carrier) + 0.00206818 * I * Ar_(carrier)T = −417.125 + 3.7875 * Ar + 61.5625 * H₂ + 0.729545 * I + 51.25 * Ar_(carrier) − 0.380208 * Ar * H₂ − 0.00244318 * Ar * I − 0.0625 * Ar * Ar_(carrier) − 0.0260417 * H₂ * I − 6.77083 * H₂ * Ar_(carrier) − 0.0352273 * I * Ar_(carrier)

In these equations:

-   -   I is expressed in amperes.    -   The gas flow rates Ar, Ar_(carrier) and H2 are expressed in        liters per minute normalized to atmospheric pressure.    -   The supply parameters have the same orders of priority in each        equation, and are thus taken in the following order of        decreasing priority: Ar, H2, I, Ar_(carrier). This identity of        the orders of priority is associated only with this torch and        does not apply as a general rule.

In practice, the operation of the torch is limited only by the maximumdissipated power, namely 55 kW. If a safety margin of 10 kW is adopted,then the torch will no longer be used above 45 kw and the arc intensitywill be conditioned by the following formula:I≦45000/VV being the voltage of the plasma arc as expressed in volts and given tothe computer 100 by the control unit 30 via the connection 120 betweenthe control unit 30 and the computer 100.

The minimum arc intensity, as well as the normal operating ranges of theother supply parameters, that is to say Ar, H₂, and Ar_(carrier),correspond to the scopes in which these equations are valid.

For example:

-   -   If Imax needs to be reduced, then Ar will be increased by a        value equal to its step size since the coefficient of Ar in this        equation is negative and equal to −1.51175. But if Imax needs to        be increased, conversely, then Ar will be reduced by a value        equal to its step size.    -   If Ar comes out of its normal operating range and if Imax needs        to be reduced, then H2 will be reduced by a value equal to its        step size since the coefficient of H₂ in this equation is        positive and equal to +38.2083. If Imax needs to be increased,        conversely, then H2 will be increased by a value equal to its        step size.

In a first numerical example, the deposit is CuNiIn (copper, nickel andIndium) and it needs to have an oxide level at most equal to 2%.Observations have shown that the level of oxides expressed as apercentage, that is to say a value ranging from 0% to 100%, is given bythe following formula:oxide level=0.0163213*I _(max)+0.00778653*Twith the variable I_(max) having priority over the variable T, the modelcorresponding to this deposit therefore containing the aforementionedequations giving Imax and T.

The optimum ranges and the acceptable ranges of I_(max) and T, eachexpressed by a minimum value and a maximum value, are as follows: SprayOrder of Acceptable ranges Optimum ranges characteristics prioritymin/max min/max I_(max) [0, 255] 1  0/40  0/20 T (° C.) 2 190/280190/220

The initial values of the supply parameters and the normal operatingranges, expressed in terms of a minimum/maximum value, are given by thefollowing table: Supply Initial Operating ranges Correction stepparameters values min/max size I (A) 450 360/540 ±10 Ar (L/mn) 45 36/54±1 H₂ (L/mn) 15 12/18 ±0.5 Ar_(carrier) (L/mn) 2.5 2/3 ±0.1

In a second numerical example, the deposit is to have a hardness atleast equal to 120 Hv, this deposit being formed by using theaforementioned torch and deposit composition. Experiments have shownthat the hardness expressed in Hv is given by the following formula:Hardness=8.4*L+5.2*I _(max)

Since the variable L has the greatest effect, the operator willconsequently use the following equation system in which L has priorityover I_(max):L = 17.9632 − 0.30375 * Ar − 0.377083 * H₂ − 0.00725 * I − 0.025 * Ar_(carrier) + 0.0107292 * Ar * H₂ + 0.000126136 * Ar * I + 0.04675 * Ar * Ar_(carrier) − 0.0000473485 * H₂ * I + 0.0395833 * H₂ * Ar_(carrier) + 0.00206818 * I * Ar_(carrier)I  max  = −45.2957 − 1.51175 * Ar + 38.2083 * H₂ + 0.234739 * I − 8.94 * Ar_(carrier) − 0.39724 * ArH₂ − 0.00272557 * Ar * I + 1.04463 * Ar * Ar_(carrier) + 0.0170028 * H₂ * I − 6.46563 * H₂ * Ar_(carrier) − 0.0231932 * I * Ar_(carrier)

The optimum ranges and the acceptable ranges of L and I_(max), expressedin terms of min/max values, are as follows: Spray Order of Acceptableranges Optimum ranges characteristics priority min/max min/max L (mm) 1 2/9.8 2/5  I_(max) [0, 255] 2 20/180 20/100

The initial values of the supply parameters and the normal operatingranges, expressed in terms of a min/max value, are given by thefollowing table: Supply Initial Operating ranges Correction stepparameters values min/max size I (A) 450 360/540 ±10 Ar (L/mn) 45 36/54±1 H₂ (L/mn) 15 12/18 ±0.5 Ar_(carrier) (L/mn) 2.5 2/3 ±0.1

This third numerical example combines the two preceding examples, thedeposit needing to have a level of oxides at most equal to 2% and ahardness at least equal to 120 Hv, this deposit being formed by usingthe aforementioned torch and deposit composition. Experiments have shownthat the level of oxides expressed as a percentage, that is to say from0% to 100%, and the hardness expressed in Hv are given by the followingformula:level of oxides=0.0163213*Imax+0.00778653*THardness=8.4*L+5.2*I _(max)

Here, the operator uses the following system of equations in which Imaxhas priority over L and L has priority over T:I_(max) = −45.2957 − 1.51175 * Ar + 38.2083 * H₂ + 0.234739 * I − 8.94 * Ar_(carrier) − 0.39724 * ArH₂ − 0.00272557 * Ar * I + 1.04463 * Ar * Ar_(carrier) + 0.0170028 * H₂ * I − 6.4563 * H₂ * Ar_(carrier) − 0.0231932 * I * Ar_(carrier)L = 17.9632 − 0.30375 * Ar − 0.377083 * H₂ − 0.00725 * I − 0.025 * Ar_(carrier) + 0.0107292 * Ar * H₂ + 0.000126136 * Ar * I + 0.04675 * Ar * Ar_(carrier) − 0.0000473485 * H₂ * I + 0.0395833 * H₂ * Ar_(carrier) + 0.002068118 * I * Ar_(carrier)T = −417.125 + 3.7875 * Ar + 61.5625 * H₂ + 0.729545 * I + 51.25 * Ar_(carrier) − 0.380208 * Ar * H₂ − 0.00244318 * Ar * I − 0.0625 * Ar * Ar_(carrier) − 0.0260417 * H₂ * I − 6.77083 * H₂ * Ar_(carrier) − 0.0352273 * I * Ar_(carrier)

The optimum ranges and the acceptable ranges of I_(max), L and T,expressed in terms of min/max values, are as follows: Spray Order ofAcceptable ranges Optimum ranges characteristics priority min/maxmin/max I_(max) [0, 255] 1 20/40 19/20 L (mm) 2   2/9.8 2/5 T (° C.) 3190/280 190/220

The initial values of the supply parameters and the normal operatingranges, expressed in terms of a min/max value, are given by thefollowing table: Supply Initial Operating ranges Correction stepparameters values min/max size I (A) 450 360/540 10 Ar (L/mn) 45 36/54 1H₂ (L/mn) 15 12/18 0.5 Ar_(carrier) (L/mn) 2.5 2/3 0.1

In a fifth numerical example, the residual stresses of the deposit areto be compressive and limited to −400 MPa (megapascal), this depositbeing formed by using the aforementioned torch and deposit composition.Experiments have shown that the residual stress is given by thefollowing formula:Stress_(Mpa)=720.92−2.5342*T

The operator uses a single equation here, namely the one for T:T = −417.125 + 3.7875 * Ar + 61.5625 * H₂ + 0.729545 * I + 51.25 * Ar_(carrier) − 0.3802008 * Ar * H₂ − 0.00244318 * Ar * I − 0.0625 * Ar * Ar_(carrier) − 0.0260417 * H₂ * I − 6.77083 * H₂ * Ar_(carrier) − 0.0352273 * I * Ar_(carrier)

The optimum ranges and the acceptable ranges of L and T, expressed interms of min/max values, are as follows: Spray Acceptable ranges Optimumranges characteristics min/max min/max T (° C.) 280/360° C. 280/300° C.

The initial values of the supply parameters and the normal operatingranges, expressed in terms of a min/max value, are given by thefollowing table: Supply Initial Operating ranges Correction stepparameters values min/max size I (A) 450 360/540 10 Ar (L/mn) ??? 36/541 H₂ (L/mn) ??? 12/18 0.5 Ar_(carrier) (L/mn) ??? 2/3 0.1

A deposit without cracks is desired in a fourth numerical example, thisdeposit being formed by using the torch and a deposit pf WCCo (tungstencobalt carbide). Experiments have shown that the number of cracks permm² is given by the following formula:Number of cracks=−0.22+0.5*P+0.00009*I _(max)

In this formula, a number of cracks less than zero means that there areno cracks.

Here, the operator uses the following system of equations in which P haspriority Imax, P having a preponderant effect and I_(max) having asecondary effect:P = −7.85889 + 0.0795898 * Ar − 0.0244141 * H₂ + 0.00776811 * I + 2.22168 * Ar_(carrier) − 0.00712077 * ArH₂ − 0.000521573 * Ar * I − 0.0266113 * Ar * Ar_(carrier) − 0.000616599 * H₂ * I + 0.10376 * H₂ * Ar_(carrier) − 0.000998757 * I * Ar_(carrier)I_(max) = −45.2957 − 1.51175 * Ar + 38.2083 * H₂ + 0.234739 * I − 8.94 * Ar_(carrier) − 0.39724 * ArH₂ − 0.00272557 * Ar * I + 1.04463 * Ar * Ar_(carrier) + 0.0170028 * H₂ * I − 6.46563 * H₂ * Ar_(carrier) − 0.0231932 * I * Ar_(carrier)

The optimum ranges and the acceptable ranges of P and I_(max), expressedin terms of min/max values, are as follows: Spray Order of Acceptableranges Optimum ranges characteristics priority min/max min/max P (mm) 1 −5/1.2 −5/1  I_(max) [0, 255] 2  20/100 20/50

The initial values of the supply parameters and the normal operatingranges, expressed in terms of a min/max value, are given by thefollowing table: Supply Initial Operating ranges Correction parametersvalues min/max step size I (A) 650 520/780 10 Ar (L/mn) 45 36/54 1 H₂(L/mn) 120  96/144 0.5 Ar_(carrier) (L/mn) 2.3 1.8/2.8 0.1

The invention thus makes it possible to guarantee a plurality ofcharacteristics of the deposit simultaneously, if the ranges of spraycharacteristics established for each characteristic of the depositoverlap. If these ranges do not overlap, then it is necessary toincrease them and tolerate a greater dispersion in some of thecharacteristics of the deposit.

The invention can be readily implemented with a commercially availablemicrocomputer equipped with suitable interfaces for collecting themeasurements of the spray characteristics and for transmitting newvalues of the supply parameters to the control unit. Other equivalentcomputing architectures are possible and do not depart from the scope ofthe invention. For example, the computing means may be those of aworkstation shared by a plurality of machines. On the other hand, it isalso possible to calculate the measurements on a first computer, forexample one which is onboard with the sensors, and to carry out theprocessing operations on a second computer, for example one which isincluded in the control unit.

It will be understood that the invention may be applied to any type ofthermal spray torch, since the measurements used for the management arecarried out on the effects of the torch, in the case in point on the jetwhich it produces and on the temperature of the deposit.

It will also be understood that the software fulfilling the functionsdescribed and claimed in this patent application may be written indifferent ways with different algorithms, without the instrumentdeparting from the scope of the invention.

It will also be understood that the proposed database is the preferredembodiment of the invention, but is not indispensable. For instance, amore rudimentary solution may also be envisaged which consists inentering the data necessary for a thermal spraying operation into thecomputer on each occasion.

The proposed example of an information system is simple and makes itpossible to organize the information necessary for a thermal sprayingoperation. More elaborate models which limit the repetition ofinformation may also be envisaged.

Sometimes, it may be necessary to attach the operating range, thecorrection step size or the order of priority to the equation/supplyparameter relation, but the proposed examples do not require this.

It will also be understood that the sensors must be able to monitor thethermal spraying as it is being carried out. In the event that the torchis mobile, these sensors will advantageously be attached to the torch,although they may also monitor the movements of the torch by othermeans. The claims also cover the case of an installation in which thetorch is fixed and the article to be coated moves in front of the torch.

1. A thermal spray instrument (10) having a thermal spray torch (12),the torch (12) having a geometrical axis (14), the torch (12) beingcapable of spraying a jet (16) along its geometrical axis (14), the jet(16) consisting of a gas flow at elevated temperature loaded with moltenparticles of the material to be sprayed, the instrument (10) having acontrol unit (30) supplying the torch (12) with ingredients by applyingthe supply parameters (122) which are communicated to it, the instrument(10) having a computer (100) communicating the supply parameters (122)to the control unit (30) by means of a unit-computer connection (120),the instrument (10) having sensors (52) capable of monitoring themovements of the torch (12), the sensors (52) being capable oftransmitting information (112, 114) about the operation of the torch(12) to the computer (100), this transmission being carried out by meansof the sensors-computer connection (100), characterized in that: a. thecomputer (100) has software for realtime analysis of the information(112, 114), in order to repetitively deduce therefrom the measurement ofat least one so-called “spray” parameter so as to find when thismeasurement is stabilized, in order to “process” this spraycharacteristic, that is to say in order to calculate a new supplyparameter value (122) and transmit it to the control unit (30) when themeasured value of the spray characteristic is outside a preestablishedso-called “acceptable” range of values specific to the spraycharacteristic being processed, this new value of the supply parameterbeing suitable for returning the spray characteristic to its acceptablerange, b. the sensors 52 also have an optical pyrometer (70) capable ofremotely measuring the thermal radiation at the surface of some article(22) to be coated, which is positioned in front of the torch (12), thepyrometer (70) having a narrow field, the pyrometer being positioned sothat the field comes as close as possible to the jet (16) on the article(22), but without interfering with this jet (16), the pyrometer (70)also being capable of periodically transmitting the temperaturemeasurement to the computer (100) by means of the sensors-computerconnection (110), the temperature measurement transmitted to thecomputer being referenced (114); c. the computer (100) is capable ofcorrecting the temperature measurement (114) as a function of theemissivity coefficient of the coating (22), this corrected temperaturecalled deposit temperature T constituting the spray characteristic. 2.The instrument as claimed in claim 1, characterized in that: d. thesensors (52) have a camera (54) capable of periodically providing thecomputer (100) with the information (112, 114) in the form of digitalimages (112) of the jet (16) as seen in profile over a part of itslength, e. the computer (100) also measures from the image (112) andprocesses the width L of the jet (16), L also constituting a sprayparameter, an order of priority been defined in the processing of thespray characteristics, the processing of the temperature T being givenpriority however, the camera (54) being capable of the observing the jet(16) with a resolution at least equal to 0.5 mmm, L being proportionalto the standard deviation of the distribution of the luminance of thejet (16) along a geometrical line (154) transverse to the jet (16), inorder also to regulate the hardness of the coating (22).
 3. Theinstrument as claimed in claim 1, characterized in that: d. the sensors(52) have a camera (54) capable of periodically providing the computer(100) with the information (112, 114) in the form of digital images(112) of the jet (16) seen in profile over a part of its length, e. thecomputer (100) also measures from the image (112) and processes theposition (P) of the jet (16), P also consistuting a spraycharacteristic, an order of priority being defined in the processing ofthe spray characteristics, the processing of the temperature T stillbeing given the highest priority, the camera (54) being capable ofobserving the jet (16) with a resolution at least equal to 0.5 mmm, Pbeing, to within a constant value P₀, the mean of the distribution ofthe luminance of the jet (16) along a geometrical line (154) transverseto the jet (16) so as also to regulate the level of cracks of thecoating (22).
 4. The instrument as claimed in claim 1 to 3,characterized in that: d. the sensors (52) have a camera (54) capable ofperiodically providing the computer (100) with the information (112,114) in the form of digital images (112) of the jet (16) seen in profileover a part of its length, e. the computer (100) measures from the image(112) and processes the maximum intensity I_(max) of the jet (16),I_(max) also constituting a spray characteristic, an order of prioritybeing defined in the processing of the spray characteristics, theprocessing of the temperature T still being given the highest priority.5. The instrument as claimed in claim 1, characterized in that thecomputer is capable of emitting a warning signal when a spraycharacteristic is outside its acceptable range and it cannot calculate anew supply parameter value without making the value of the supplyparameter depart from a so-called “normal operating” rangepreestablished for this supply parameter.
 6. The instrument as claimedin claim 1, characterized in that the thermal spraying operation isinterrupted when a spray characteristic is outside its acceptable rangeand the computer (100) cannot calculate a new supply parameter valuewithout making the value of the supply parameter depart from a so-called“normal operating” range preestablished for this supply parameter. 7.The instrument as claimed in claim 1, characterized in that the computer(100) is capable of: identifying the situation according to which allthe spray characteristics being used are each in their preestablishedacceptable range, in calculating a new supply parameter value (122) andtransmitting it to the control unit (30) when the measured value of thespray characteristic is outside a preestablished so-called “optimum”range of values specific to the spray characteristic being processed,this optimum range being included in the acceptable range, this newvalue of the supply parameter being suitable for returning the spraycharacteristic to its acceptable range.
 8. The instrument as claimed inclaim 1, characterized in that the camera (52) has a charge accumulationmatrix.
 9. The instrument as claimed in claim 1, characterized in thatthe camera is capable of providing images of the jet (112) with aresolution at least equal to 0.1 mm.
 10. The instrument as claimed inclaim 1, with a flame (17) emerging from the torch (12) in the vicinityof the apex of the cone formed by the jet (16), characterized in thatthe camera (54) is positioned in order to provide images (122) of thejet (16) downstream of the flame (17).
 11. The instrument as claimed inclaim 1, with the torch (12) having an injector (18) so as to spraypowder, this injection taking place at the outlet of the torch (12)along a geometrical axis (20) substantially perpendicular to thegeometrical axis of the torch (14), characterized in that the camera(54) is positioned in order to see the jet (16) along a geometrical axis(56) substantially orthogonal to the geometrical axis (20) of theinjector (18).
 12. The instrument as claimed in claim 1, characterizedin that the luminance of the jet (16), as taken into account in theprocessing, is the light level of the pixels of the images (112). 13.The instrument as claimed in claim 1, characterized in that the computer(100) is capable of taking into account only the pixels of the images(112) whose light level is higher than a preestablished value.
 14. Theinstrument as claimed in claim 1, characterized in that the maximumluminance Imax, the width L and the position P of the jet (16), as areprocessed by the computer (100), are averages over the images grouped inbatches.
 15. The instrument as claimed in claim 1, characterized in thatat least one of the measurements taken from the images (112), namely themaximum luminance I_(max), the width L and the position P of the jet(16), is deduced from a Gaussian law of the formI_(max)·exp(−(x−(P+P0))²/σ²)/2π.
 16. The instrument as claimed in claim1, characterized in that the pyrometer 70 has a laser sight (74).